Zeolitic para-xylene separation with diethyltoluene heavy desorbent

ABSTRACT

A chromatographic process able to separate para-xylene from C 8  isomers and C 9  aromatics. In the process, the para-xylene-containing feed mixture is contacted with an X or Y zeolite adsorbent having Group IA or IIA cations, e.g., barium and/or potassium at exchangeable cationic sites. The para-xylene is selectively adsorbed onto the adsorbent. The feed is then removed from the adsorbent and the para-xylene recovered by desorption with diethyltoluene. The C 9  &#39;s and the other xylene isomers in the raffinate, can be separated from this heavy desorbent by fractionation of the raffinate and the desorbent recycled to the process. The preferred desorbents are 2,3-diethyltoluene, 2,5-diethyltoluene and 2,6-diethyltoluene.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Ser. No. 197,740,filed May 23, 1988, now U.S. Pat. No. 4,864,069.

FIELD OF THE INVENTION

The field of art to which the claimed invention pertains is hydrocarbonseparation. More specifically, the invention relates to a process forseparating para-xylene from a feed mixture comprising at least twoxylene isomers, including the para-isomer, which process employs azeolitic adsorbent and particular desorbents. It is particularlyadvantageous in a process in which the feed also contains C₉ aromatichydrocarbons.

BACKGROUND OF THE INVENTION

In numerous processes described in the patent literature, for exampleU.S. Pat. Nos. 3,626,020 to Neuzil, 3,663,638 to Neuzil, 3,665,046 todeRosset, 3,668,266 to Chen et al., 3,686,342 to Neuzil et al.,3,700,744 to Berger et al., 3,734,974 to Neuzil, 3,894,109 to Rosback,3,997,620 to Neuzil and B426,274 to Hedge, particular zeoliticadsorbents are used to separate the para isomer of dialkyl substitutedmonocyclic aromatics from the other isomers, particularly para-xylenefrom other xylene isomers. Many of the above patents use benzene,toluene, or p-diethylbenzene as the desorbent. P-diethylbenzene (p-DEB)has become a commercial standard for this separation. However, p-DEB isa "heavy" desorbent (higher boiling than p-xylene) which suffers in theprocess for separating p-xylene from feed mixtures containing C₉aromatics because the boiling point of p-DEB is too close to the boilingpoint of C₉ aromatics in the feed. Because the C₉ aromatics aredifficult to separate from p-DEB by simple fractionation, the C₉aromatics would gradually build up in the desorbent, which must berecycled for economic reasons. In the commercial process for recoveringp-xylene from feed mixtures containing isomers, therefore, it has beennecessary to reduce C₉ aromatics in the feed to below about 0.1% priorto the adsorptive separation of p-xylenes. This is usually done bydistillation in a so-called xylene splitter column. Of course,substantial costs associated with this practice, such as capital costsof the xylene splitter and utilities necessary to achieve substantiallycomplete removal of the C₉ aromatics, could be reduced greatly oreliminated if it were not necessary to first remove C₉ aromatics. Thus,while U.S. Pat. No. 3,686,342, supra, mentions other substitutedbenzenes as possible heavy desorbents for the para-xylene separationprocess, that reference clearly states that p-DEB is the best desorbentfor the separation and, further, does not address the problem that thepreferred desorbents may have in separating feeds containing C₉aromatics. Therefore, a higher boiling point material, that meets theselectivity requirements for desorbents and can be separated from C₉aromatics, has been long sought and is still desirable.

It is also known that crystalline aluminosilicates or zeolites are usedin adsorption separations of various mixtures in the form ofagglomerates having high physical strength and attrition resistance.Methods for forming the crystalline powders into such agglomeratesinclude the addition of an inorganic binder, generally a clay comprisinga silicon dioxide and aluminum oxide, to the high purity zeolite powderin wet mixture. The blended clay zeolite mixture is extruded intocylindrical type pellets or formed into beads which are subsequentlycalcined in order to convert the clay to an amorphous binder ofconsiderable mechanical strength. As binders, clays of the kaolin type,water permeable organic polymers or silica are generally used.

The invention herein can be practiced in fixed or moving adsorbent bedsystems, but the preferred system for this separation is acountercurrent simulated moving bed system, such as described inBroughton U.S. Pat. No. 2,985,589, incorporated herein by reference.Cyclic advancement of the input and output streams can be accomplishedby a manifolding system, which are also known, e.g., by rotary discvalves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipmentutilizing these principles are familiar, in sizes ranging from pilotplant scale (deRosset U.S. Pat. No. 3,706,812) to commercial scale inflow rates from a few cc per hour to many thousands of gallons per hour.

The invention may also be practiced in a cocurrent, pulsed batchprocess, like that described in U.S. Pat. No. 4,159,284.

Also, in some cases illustrated herein, it is necessary to remove threeproduct streams in order to obtain a desired product intermediate inadsorption strength from an extract and a raffinate stream. Thisintermediate stream can be termed a second raffinate stream, as in U.S.Pat. No. 4,313,015 or a second extract stream, as in U.S. Pat. No.3,723,302, both incorporated herein by reference, the latterincorporating abandoned application Ser. No. 100,105 filed Dec. 21,1970. This case pertains when a contaminating component in the feed,such as p-ethyltoluene, is more strongly adsorbed than the desiredproduct, p-xylene. It is not always necessary to remove p-ethyltoluenefrom p-xylene, e.g., where terephthalic acid is the final product of theoxidation of p-xylene, since oxidation of p-ethyltoluene results in thesame product. However, if it is desired to keep the concentration of thecontaminating component in the product as low as possible, a firstextract is taken off, high in concentration of the desired component andlower in the contaminating product followed by a second extractwithdrawn at a point in zone 3 between the desorbent inlet and the firstextract point, containing a high concentration of the contaminant and alower concentration of the desired product. It is not necessary,however, to use a second desorbent, if the desorbent is able to firstdesorb the lightly held product and then desorb the remaining morestrongly held contaminants, as disclosed in the aforementioned abandonedapplication. If the contaminating component in high concentrations andpurity is desired, this can be achieved by withdrawing a second extractin the cocurrent pulsed batch process mentioned above.

The functions and properties of adsorbents and desorbents in thechromatographic separation of liquid components are well-known, but forreference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporatedherein.

I have discovered a process for employing a zeolite adsorbent for theseparation of p-xylene from its isomers and, particularly, a desorbentwhich is a substantial improvement in a process for separating xyleneisomers where the feed mixture also contains C₉ aromatic impurities.

SUMMARY OF THE INVENTION

In brief summary, the invention is a chromatographic process forseparating p-xylene from a feed mixture comprising p-xylene and C₉aromatic hydrocarbons and optionally, one or more additional xyleneisomers (including ethylbenzene) comprising contacting said feed mixturewith an X- or Y-type zeolite exchanged with Group IA or IIA metal ionsat exchangeable cationic sites to effect the selective adsorption ofsaid p-xylene and produce a raffinate comprising the other xyleneisomers, including ethylbenzene and C₉ aromatics. P-xylene is recoveredby contacting the adsorbent with a desorbent comprising diethyltoluene(DET). As used herein, diethyltoluene is intended to signify each of theisomers and any mixture thereof. The preferred individual isomers are2,3-DET, 2,5-DET and 2,6 DET since they, or mixtures of any of thesethree, provide greater selectivity for p-xylene and higher desorptionrates in the separation process. A preferred desorbent will contain atleast about 40% (wt.) and up to about 98% (wt.) of one or more of thepreferred isomers. The desorbent is higher boiling (e.g.,3,5-diethyltoluene: b.p.=205° C.) than the C₉ aromatics, making itpossible to separate the C₉ 's from the desorbent by simplefractionation so that the desorbent can be reused in the process withoutbuilding up C₉ aromatics in the recycled desorbent. The invention, inanother aspect, is a process for separating C₉ aromatics from a feedmixture of C₉ aromatics and p-xylene.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chromatographic representation of the separation of p-xylenefrom a mixture of xylene isomers and C₉ aromatics with a K-exchanged Yzeolite and a desorbent comprising a 30/70 mixture of diethyltoluene andn-heptane.

FIG. 2 is similar to FIG. 1 except that the adsorbent is BaX zeolite andthe desorbent is 100% diethyltoluene isomers.

FIG. 3 shows the separation of the isomers of diethyltoluene with BaXzeolite using p-xylene as desorbent.

FIGS. 4 and 5 are similar to FIG. 2 except that the desorbents aredifferent mixtures of individual DET isomers.

DETAILED DESCRIPTION OF THE INVENTION

Adsorbents to be used in the process of this invention comprise specificcrystalline aluminosilicates or molecular sieves, namely X and Yzeolites. The zeolites have known cage structures in which the aluminaand silica tetrahedra are intimately connected in an openthree-dimensional network to form cage-like structures with window likepores. The tetrahedra are cross-linked by the sharing of oxygen atomswith spaces between the tetrahedra occupied by water molecules prior topartial or total dehydration of this zeolite. The dehydration of thezeolite results in crystals interlaced with cells having moleculardimensions and thus, the crystalline aluminosilicates are often referredto as "molecular sieves" when the separation which they effect isdependent essentially upon differences between the sizes of the feedmolecules as, for instance, when smaller normal paraffin molecules areseparated from larger isoparaffin molecules by using a particularmolecular sieve. In the process of this invention, however, the term"molecular sieves", although widely used, is not strictly suitable sincethe separation of specific aromatic isomers is apparently dependent ondifferences in electrochemical attraction of the different isomers andthe adsorbent rather than on pure physical size differences in theisomer molecules.

In hydrated form, the crystalline aluminosilicates encompass type Xzeolites which are represented by Formula 1 below in terms of moles ofoxides:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :(2.5±0.5)SiO.sub.2 :yH.sub.2 OFormula 1

where "M" is a cation having a valence of not more than 3 which balancesthe electrovalence of the tetrahedra and is generally referred to as anexchangeable cationic site, "n" represents the valence of the cation,and "y", which represents the moles of water, is a value up to about 9depending upon the identity of "M" and the degree of hydration of thecrystal. As noted from Formula 1, the SiO₂ /Al₂ O₃ mole ratio is2.5±0.5. The cation "M" may be monovalent, divalent or trivalent cationsor mixtures thereof.

The type Y structured zeolite, in the hydrated or partially hydratedform, can be similarly represented in terms of moles of oxides as inFormula 2 below:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :wSiO.sub.2 :yH.sub.2 OFormula 2

where "M", "n" and "y" are the same as above and "w" is a value greaterthan about 3 up to about 6. The SiO₂ /Al₂ O₃ mole ratio for type Ystructured zeolites can thus be from about 3 to about 6. For bothzeolites, the cation "M" may be one or more of a variety of cations but,as the zeolites are initially prepared, the cation "M" is also usuallypredominately sodium. The type Y zeolite containing predominately sodiumcations at the exchangeable cationic sites is, therefore, referred to asa sodium-exchanged type-Y, or NaY, zeolite. Depending upon the purity ofthe reactants used to make the zeolite, other cations mentioned abovemay be present, however, as impurities.

The zeolites useful in the invention are typical as described above.However, the exchange of the cation of the as-manufactured zeolite byions from Group IA or IIA, e.g., barium or potassium or mixturesthereof, is necessary to achieve the separation.

Typically, adsorbents used in separative processes contain thecrystalline material dispersed in an amorphous, inorganic matrix orbinder, having channels and cavities therein which enable liquid accessto the crystalline material. Silica, alumina, clay or mixtures thereofare typical of such inorganic matrix materials. The binder aids informing or agglomerating the crystalline particles which otherwise wouldcomprise a fine powder. The adsorbent may thus be in the form ofparticles such as extrudates, aggregates, tablets, macrospheres orgranules having a desired particle range, preferably from about 16 toabout 60 mesh (Standard U.S. Mesh) (1.9 mm to 250 micron).

Feed mixtures which can be utilized in the process of this inventionwill comprise para-xylene, at least one other C₈ aromatic isomer, andmay also contain one or more C₉ aromatics as impurities. Mixturescontaining substantial quantities of para-xylene and other C₈ aromaticisomers and C₉ aromatics generally are produced by reforming andisomerization processes, processes which are well known to the refiningand petrochemical arts. Many of the C₉ aromatics have boiling points inthe range of 160°-170° C. and cannot be easily removed by distillationfrom the standard desorbent, p-diethylbenzene. In the current process,therefore, the C₉ 's are usually removed from the feed by distillationbefore the adsorptive separation and consequent contact with the normaldesorbent. I have discovered a desorbent which can be easily separatedfrom the C₉ aromatics by fractionation and does not require the largecolumn and quantity of utilities to pretreat the feed, resulting insubstantial cost savings.

Reforming processes can provide feed mixtures for the process of thisinvention. In reforming processes, a naphtha feed is contacted with aplatinum-halogen-containing catalyst at severities selected to producean effluent containing C₈ aromatic isomers. Generally, the reformate isthen fractionated to concentrate the C₈ aromatic isomers in a C₈fraction which will contain the C₈ aromatic isomers as well as C₈nonaromatics and C₉ aromatics. Feed mixtures for the process of thisinvention may also be obtained from isomerization and transalkylationprocesses. Xylene mixtures which are deficient in one or more isomerscan be isomerized, at isomerization conditions, to produce an effluentcontaining the C₈ aromatic isomers, e.g., enriched in p-xylene, as wellas C₈ nonaromatics and C₉ aromatics. The C₉ aromatic content ofisomerized xylene isomers can be as much as 1-2% depending onisomerization conditions. Likewise, transalkylation of mixtures of C₇and C₉ aromatics produces xylene isomers which contain C₉ aromatics. Inall of these catalytic processes, the xylene splitter column must beemployed to remove C.sub. 9 aromatics from C₈ aromatics beforeconventional adsorptive xylene separation methods can be employed. Thus,the feed mixtures to the process of this invention can containquantities of C₉ aromatics and may also contain quantities of straightor branched chain paraffins, cycloparaffins, or olefinic material. It ispreferable to have these quantities at a minimum amount in order toprevent contamination of products from this process by materials whichare not selectively adsorbed or separated by the adsorbent. Preferably,the above-mentioned contaminants should be less than about 20% of thevolume of the feed mixture passed into the process.

To separate the para-xylene from a feed mixture containing para-xylene,at least one other C₈ aromatic and C₉ aromatics, the mixture iscontacted with the adsorbent at adsorption conditions and thepara-xylene (and para-ethyltoluene, if present) is more selectivelyadsorbed and retained by the adsorbent while the other components arerelatively unadsorbed and are removed from the interstitial void spacesbetween the particles of adsorbent and from the surface of theadsorbent. The adsorbent containing the more selectively adsorbedpara-xylene is referred to as a "rich" adsorbent--rich in the moreselectively adsorbed para-xylene. The para-xylene is then recovered fromthe rich adsorbent by contacting the rich adsorbent with a desorbentmaterial at desorption conditions.

In this process, which employs zeolitic adsorbents and which isgenerally operated continuously at substantially constant pressures andtemperatures to ensure liquid phase, the desorbent material relied uponmust be judiciously selected to satisfy several criteria. First, thedesorbent material should displace an extract component from theadsorbent with reasonable mass flow rates without itself being sostrongly adsorbed as to unduly prevent the extract component fromdisplacing the desorbent material in a following adsorption cycle.Secondly, the desorbent material must be compatible with the particularadsorbent and the particular feed mixture. More specifically, they mustnot reduce or destroy the critical selectivity of the adsorbent for anextract component with respect to the raffinate component or reactchemically with the feed components. Desorbent materials shouldadditionally be easily separable from the feed mixture that is passedinto the process. Both the raffinate components and the extractcomponents are typically removed from the adsorbent in admixture withdesorbent material and without a method of separating at least a portionof desorbent material, the purity of the extract product and theraffinate product would not be very high nor would the desorbentmaterial be available for reuse in the process. It is, therefore,contemplated that any desorbent material used in this process will havea substantially different average boiling point than that of the feedmixture or any of its components, i.e., more than about 5° difference,to allow separation of at least a portion of the desorbent material fromfeed components in the extract and raffinate streams by simplefractional distillation, thereby permitting reuse of desorbent materialin the process.

Finally, desorbent materials should be readily available and reasonablein cost. However, a suitable desorbent or desorbents for a particularseparation with specific adsorbent are not always predictable. In thepreferred isothermal, isobaric, liquid-phase operation of the process ofthis invention, when the feed material to the separation processcontains more than about 0.1% C₉ aromatics, I have found that adesorbent material comprising diethyltoluene (individual isomers ormixtures thereof) will desorb the extract from the adsorbent and can beseparated from C₉ 's by distillation. It has been found that three ofthe possible isomers of diethyltoluene have improved ability to desorbp-xylene. Those isomers are 2,3-DET, 2,5-DET and 2,6-DET. The preferreddesorbent material for this separation is a mixture of individualisomers containing a total of at least about 40% (wt.) of one or more ofthe preferred isomers, 2,3-DET, 2,5-DET or 2,6-DET.

Adsorption conditions will include a temperature range of from about 20°to about 250° C. with about 60° to about 200° C. being more preferredand a pressure just sufficient to maintain liquid phase, which may befrom about atmospheric to 600 psig. Desorption conditions will includethe same range of temperatures and pressure as used for adsorptionconditions.

A dynamic testing apparatus is employed to test various adsorbents anddesorbent material with a particular feed mixture to measure theadsorbent characteristics of adsorptive capacity and exchange rate. Theapparatus consists of a helical adsorbent chamber of approximately 70 ccvolume having inlet and outlet portions at opposite ends of the chamber.The chamber is contained within a temperature control means and, inaddition, pressure control equipment is used to operate the chamber at aconstant predetermined pressure. Quantitative and qualitative equipment,such as refractometers, polarimeters, chromatographs, etc., can beattached to the outlet line of the chamber and used to analyze,"on-stream", the effluent stream leaving the adsorbent chamber.

A pulse test, performed using this apparatus and the following generalprocedure, is used to determine data, e.g., selectivities, for variousadsorbent systems. The adsorbent is filled to equilibrium with aparticular desorbent by passing the desorbent material through theadsorbent chamber. At a convenient time, a pulse of feed containingknown concentrations of a tracer and of a particular extract componentor of a raffinate component, or both, all diluted in desorbent materialis injected for a duration of several minutes. Desorbent flow isresumed, and the tracer and the extract and raffinate components areeluted as in a liquid-solid chromatographic operation. The effluent canbe analyzed by on-stream chromatographic equipment and traces of theenvelopes of corresponding component peaks developed. Alternatively,effluent samples can be collected periodically and later analyzedseparately by gas chromatography.

From information derived from the test, adsorbent performance can berated in terms of void volume, retention volume for an extract or araffinate component, and the rate of desorption of an extract componentfrom the adsorbent and selectivity. Void volume is the non-selectivevolume of the adsorbent, which is expressed by the amount of desorbentpumped during the interval from initial flow to the center of the peakenvelope of the tracer. The net retention volume of an extract or araffinate component may be characterized by the distance between thecenter of the peak envelope (gross retention volume) of the extract orraffinate component and the center of the peak envelope (void volume) ofthe tracer component or some other known reference point. It isexpressed in terms of the volume in cubic centimeters of desorbentmaterial pumped during this time interval represented by the distancebetween the peak envelopes. The rate of exchange or desorption rate ofan extract component with the desorbent material can generally becharacterized by the width of the peak envelopes at half intensity. Thenarrower the peak width, the faster the desorption rate. The desorptionrate can also be characterized by the distance between the center of thetracer peak envelope and the disappearance of an extract component whichhas just been desorbed. This distance is again the volume of desorbentmaterial pumped during this time interval. Selectivity, β, is determinedby the ratio of the net retention volumes of the more strongly adsorbedcomponent to each of the other components.

The following non-limiting examples are presented to illustrate theprocess of the present invention and are not intended to unduly restrictthe scope of the claims attached hereto.

EXAMPLE I

In this experiment, a pulse test, using the apparatus as describedabove, was performed to evaluate the ability of the present invention toseparate para-xylene (b.p. 138° C.) from the other xylene isomers andethylbenzene (b.p's. from 136°-145° C.) and from C₉ aromatics. Theadsorbent used was a Y faujasite exchanged with potassium, dried at400°-450° C., combined with 15 wt. % of an amorphous clay binder.

For each pulse test, the column was maintained at a temperature of 150°C. and at a pressure of 165 psig so as to maintain liquid-phaseoperations. Gas chromatographic analysis equipment was attached to thecolumn effluent stream in order to determine the composition of theeffluent material at given time intervals. The feed mixture employed foreach test was 5 cc of a mixture containing 0.45 cc each of the xyleneisomers and ethylbenzene and each of the following C₉ aromatics: cumene,n-propylbenzene, p-ethyltoluene, mesitylene, 1,2,4-trimethylbenzene and1,2,3-trimethylbenzene. Normal nonane (0.45 cc) was used as a tracer and4.95 cc desorbent material was added to the feed. The desorbent materialcomprised 30 vol. % of a diethyltoluene (DET) isomer mixture with theremainder being n-C₇ paraffin. The DET isomer distribution of thedesorbent was that of mixture A, Table 1.

                  TABLE 1                                                         ______________________________________                                        DESORBENT ISOMER DISTRIBUTION (DET BASIS)                                                      A    B                                                       ______________________________________                                        3,5-DET:           48.4   18.2                                                2,4- + 3,4-DET:     1.0    3.9                                                2,3-DET:            7.5    7.1                                                2,5-DET:           18.9   40.9                                                2,6-DET:           24.2   29.9                                                ______________________________________                                    

The operations taking place for the test were as follows: The desorbentmaterial was run continuously at a rate of about 1.2 cc per minute. Atsome convenient time interval, the desorbent was stopped and the feedmixture was run for a 4.2 minute interval. The desorbent stream was thenresumed and continued to pass into the adsorbent column until all of thefeed aromatics had been eluted from the column as determined bychromatographic analysis of the effluent material leaving the adsorptioncolumn.

The results of the tests shown in Table 2 and the chromatographictracing of FIG. 1 illustrate the invention. The table lists the grossretention volume (GRV) and net retention volume (NRV) for each componentof the feed and, β, for each component with respect to the reference,p-xylene.

In the drawings, the traces for the components are numbered as follows:n-nonane 1; mesitylene 2, m-xylene 3; o-xylene 4; 1,2,3-trimethylbenzene5; 1,2,4-trimethylbenzene 6; n-propylbenzene 7; ethylbenzene 8; cumene9; p-xylene 10; p-ethyltoluene 11.

                                      TABLE 2                                     __________________________________________________________________________                Gross  Net                                                                    Retention                                                                            Retention                                                                            Selectivity                                                                          Boiling                                      Component   Volume (ml)                                                                          Volume (ml)                                                                          (Beta) Point(°C.)                            __________________________________________________________________________    n-Nonane    46.9   0.0    0.00 (Tracer)                                       Ethylbenzene                                                                              73.8   26.9   1.86   136                                          p-Xylene    97.0   50.1   1.00 (Ref.)                                                                          138                                          Cumene      72.5   25.6   1.96   153                                          o-Xylene    59.0   12.1   4.14   144                                          n-Propylbenzene                                                                           66.9   20.0   2.50   159                                          p-Ethyltoluene                                                                            128.8  81.9   0.61   162                                          Mesitylene  53.4   6.5    7.69   163                                          1,2,4-Trimethylbenzene                                                                    66.8   19.8   2.52   168                                          1,2,3-Trimethylbenzene                                                                    61.4   14.5   3.45   175                                          m-Xylene    57.3   10.3   4.84   139                                          __________________________________________________________________________

EXAMPLE II

Another pulse test was run under the same conditions and with the samefeed mixture as Example I, except that the desorbent was 100%diethyltoluene with isomer distribution B (Table 1) and the adsorbentwas BaX.

The results of the test are shown in Table 3 and FIG. 2. The componenttraces in FIG. 2 are numbered as stated above in Example I.

                  TABLE 3                                                         ______________________________________                                                      Gross      Net                                                                Retention  Retention                                                          Volume     Volume   Selectivity                                 Component     (ml)       (ml)     (Beta)                                      ______________________________________                                        n-Nonane      41.3       0.0      0.00                                        Ethylbenzene  56.7       15.4     1.52                                        p-Xylene      64.6       23.3     1.00                                        Cumene        58.3       17.0     1.37                                        o-Xylene      48.5       7.2      3.25                                        n-Propylbenzene                                                                             50.4       9.1      2.56                                        p-Ethyltoluene                                                                              69.0       27.7     0.84                                        Mesitylene    44.0       2.6      8.80                                        1,2,4-Trimethylbenzene                                                                      47.9       6.6      3.53                                        1,2,3-Trimethylbenzene                                                                      46.8       5.4      4.28                                        m-Xylene      47.9       6.6      3.53                                        ______________________________________                                    

EXAMPLE III

In an effort to pinpoint those DET isomers which might be most effectivein separating p-xylene from its isomers, a pulse test was run, in themanner previously described, to separate the isomers of DET usingp-xylene (the desired extract of the separation process of theinvention) as the desorbent. The adsorbent was the same as used inExample II. It has previously been observed that the best desorbent fordesorbing the preferentially adsorbed component in a separation ofparticular components has a selectivity with respect to the adsorbentabout the same as, or slightly less than, the extract component. It wasreasoned, therefore, that if the individual isomers of the desorbent areseparated on the same adsorbent used to separate the xylene isomers, andthe extract component, p-xylene, of the latter separation is used as thedesorbent, the more strongly adsorbed DET isomers will be the preferredisomers in the p-xylene separation. The results of the separation of theisomers of DET with p-xylene as desorbent are shown in FIG. 3 and thefollowing Table 4, under the headings GRV (gross retention volume), NRV(net retention volume), width of peak at one half height and β(selectivity).

                  TABLE 4                                                         ______________________________________                                                     Gross      Net                                                                Retention  Retention  Selectivity                                Component    Volume (ml)                                                                              Volume (ml)                                                                              (Beta)                                     ______________________________________                                        n-C.sub.12   36.7       0.0        Tracer                                     3,5-Diethyltoluene                                                                         42.6       5.9        1.00 (Ref.)                                p-Cymene     44.3       7.6        0.78                                       2,3-Diethyltoluene                                                                         53.1       16.5       0.36                                       2,6-Diethyltoluene                                                                         51.0       14.4       0.41                                       2,5-Diethyltoluene                                                                         53.6       16.9       0.35                                       Other, DET isomer                                                                          49.3       12.7       0.47                                       Other, DET isomer                                                                          48.7       12.1       0.49                                       ______________________________________                                    

As will be seen from the data, the individual isomers, 2,3-DET, 2,5-DETand 2,6-DET are separated together as the extract and hence, havevirtually identical selectivities with respect to p-xylene.

The conclusion drawn from the above experiment was confirmed byconducting two further pulse tests, each under the same conditions andthe same adsorbent, Ba-exchanged X zeolite, to separate a mixture ofxylene isomers. In the first run, the desorbent was a 30/70 DET/n-C₇mixture having a large amount of a less effective DET isomer, 3,5-DETand approximately 52% of a mixture of the preferred isomers having thefollowing DET composition (by wt.): 46.48% 3,5 DET; 25.03% 2,6-DET;19.04% 2,5-DET; 8.10% 2,3-DET and 1.35% 2,4-DET plus 3,4-DET. In thesecond run, the concentration of preferred isomers in the DET/n-C₇mixture was increased for a total of approximately 98% (wt); theconcentration (by wt.) of the DET mixture was as follows: 1.77% 3,5-DET;13.56% 2,6-DET and 84.67% 2,5-DET. The results are shown in FIG. 4 andTable 5; the results of the second run are shown in FIG. 5 and Table 6.

                  TABLE 5                                                         ______________________________________                                                 Gross                                                                         Retention Net                                                                 Volume    Retention  Width at                                                                             Selectivity                              Component                                                                              (ml)      Volume (ml)                                                                              1/2 Height                                                                           (Beta)                                   ______________________________________                                        n-C.sub.9                                                                              36.9       0.0        6.13  Tracer                                   Ethylbenzene                                                                           74.3      37.4       15.37  1.49                                     p-Xylene 92.6      55.7       19.90  1.00 (Ref.)                              m-Xylene 58.6      21.7       12.17  2.57                                     o-Xylene 62.3      25.4       10.64  2.19                                     ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                                 Gross                                                                         Retention Net                                                                 Volume    Retention  Width at                                                                             Selectivity                              Component                                                                              (ml)      Volume (ml)                                                                              1/2 Height                                                                           (Beta)                                   ______________________________________                                        n-C.sub.9                                                                              40.8       0.0        9.84  Tracer                                   Ethylbenzene                                                                           67.6      26.8       15.04  1.69                                     p-Xylene 86.1      45.3       19.34  1.00 (Ref.)                              m-Xylene 56.9      16.0       11.54  2.82                                     o-Xylene 58.9      18.1       11.73  2.50                                     ______________________________________                                    

As seen from the results above, selectivity (β) for p-xylene wasincreased over the other xylene isomers and ethylbenzene, and the rateof desorption was increased, indicating shortened cycle time for theseparation with the preferred isomers.

In a set of experiments patterned after Example III, the same resultswere observed for K-exchanged Y zeolites, viz, 2,3-DET, 2,5-DET and2,6-DET are the preferred individual isomers for use in the invention.

What is claimed is:
 1. A process for separating p-xylene from a mixturecomprising C₉ aromatic hydrocarbons, p-xylene and at least one otherisomer of xylene which process comprises contacting said mixture with anadsorbent crystalline aluminosilicate containing a Group IA or IIA metalion at exchangeable cationic sites at adsorption conditions to effectthe selective adsorption of said p-xylene by said adsorbent and toproduce a raffinate stream comprising the less strongly adsorbed C₉hydrocarbons and said other xylene isomers and contacting said adsorbentwith a desorbent comprising at least about 40 wt. % of individualisomers selected from the group consisting of 2,3-diethyltoluene,2,5-diethyltoluene and 2,6-diethyltoluene and mixtures thereof atdesorption conditions, to effect the removal of p-xylene from saidadsorbent as an extract stream.
 2. The process of claim 1 wherein saidadsorbent is selected from the group consisting of type X and type Yzeolites.
 3. The process of claim 1 wherein a second extract stream isrecovered comprising more strongly held C₉ aromatic hydrocarbons.
 4. Theprocess of claim 2 wherein said zeolite is exchanged with potassium atexchangeable sites.
 5. The process of claim 2 wherein said zeolite isexchanged with barium at said exchangeable sites.
 6. A process forseparating p-xylene from a feed mixture comprising C₉ aromatichydrocarbons and p-xylene which process comprises contacting saidmixture with an adsorbent comprising crystalline aluminosilicatecontaining a Group IA or IIA metal ions at exchangeable cationic sitesat adsorption conditions to effect the selective adsorption of saidp-xylene by said adsorbent and to produce a raffinate stream comprisingthe less strongly adsorbed C₉ hydrocarbons and thereafter contactingsaid adsorbent with a desorbent comprising at least about 40 wt. % ofindividual isomers selected from the group consisting of2,3-diethyltoluene, 2,5-diethyltoluene and 2,6-diethyltoluene andmixtures thereof at desorption conditions to effect the removal ofp-xylene from said adsorbent as an extract stream.
 7. The process ofclaim 6 wherein said adsorbent is selected from the group consisting oftype X and type Y zeolites.
 8. The process of claim 6 wherein a secondextract stream is recovered comprising more strongly held C₉ aromatichydrocarbons.
 9. The process of claim 6 wherein said feed contains otherxylene isomers and said other xylene isomers are recovered in saidraffinate stream.
 10. In a process for separating para-xylene from afeed mixture comprising para-xylene and at least one other xyleneisomer, which process comprises contacting, at adsorption conditions,said feed with an adsorbent comprising an X- or Y-type crystallinealuminosilicate (zeolite) containing a Group IA or IIA metal atexchangeable cationic sites which selectively adsorbs said para-xylene,removing said feed from said adsorbent, and recovering said para-xyleneby desorption at desorption conditions with a desorbent material, theimprovement comprising utilizing at least about 40% (wt.) of individualisomers selected from the group consisting of 2,3-diethyltoluene,2,5-diethyltoluene and 2,6-diethyltoluene as desorbent to separate afeed additionally containing at least one C₉ aromatic hydrocarbonisomer, recovering the less strongly adsorbed C₉ aromatic feed materialsand said other xylene isomer in the raffinate and fractionating saidraffinate to recover C₉ aromatic hydrocarbons, said desorbent materialand said other xylene isomers and recycling said desorbent material tothe desorption step of said process.
 11. The process of claim 8 whereina second extract stream is recovered comprising more strongly held C₉aromatic hydrocarbons.
 12. In an improved process for the separation ofpara-xylene from a feed containing a mixture of para-xylene and at leastone other C₈ aromatic hydrocarbon, which process employs a crystallinealuminosilicate adsorbent selected from the group consisting of type Xand type Y structured zeolites containing Group IA or IIA metal cationsat the exchangeable cationic sites within said zeolite, said processcomprising the steps of:i. contacting said adsorbent with said feed; ii.removing a raffinate material, which comprises the less selectivelyadsorbed components of the feed from said adsorbent while simultaneouslyadsorbing said para-xylene and iii. contacting said adsorbent with adesorbent material at desorption conditions to effect the displacing ofsaid para-xylene from said adsorbent while simultaneously removingextract material from said adsorbent comprising desorbent andpara-xylene; the improvement which comprises employing a desorbentmaterial containing at least about 40% of individual isomers ofdiethyltoluene selected from the group consisting of 2,3-diethyltoluene,2,5-diethyltoluene and 2,6-diethyltoluene and mixtures thereof, toseparate a feed additionally containing at least one C₉ aromatichydrocarbon isomer, recovering said C₉ aromatics in the raffinate andfractionating said raffinate to recover C₉ aromatic hydrocarbon isomers,said desorbent material and said other C₈ aromatic hydrocarbon isomersand recycling said desorbent material substantially free of C₉ aromatichydrocarbons to the desorption step of said process.